his presence at the meeting looms large. >> paul manafort is a man with a lot of russian connections. he knew exactly how the russians operated. he should have known what was happening there and why that meeting was perilous. >> the question of knowingly cooperating with a foreign power to influence the election, to assist and support the campaign. that's the big question that mueller is going to try to answer through manafort. >> whatever happened before, during, and after the meeting, robert mueller now has multiple witnesses spilling secrets. rob goldstone insists he never meant to be part of a russian plot. but what he offered may provide evidence of whether any trump campaign officials wanted to join one. whatever came of the meeting, goldstone says he's living with the consequences of one very bad idea. >> are you guilty of anything? >> i'm guilty really of not standing my ground and knowing that this was a bad idea.